A steel material used for automobiles, structural materials, and the like is required to be excellent in such mechanical properties as strength, workability, and toughness. In order to improve these properties comprehensively, it is effective to make a hot-rolled steel sheet with fine crystal grains; to this end, a number of manufacturing methods to obtain a hot-rolled steel sheet which has fine crystal grains have been sought. Further, by refining crystal grains of a hot-rolled steel sheet, it is possible to manufacture a high strength hot-rolled steel sheet having excellent mechanical properties even if the amount of alloying elements added is reduced.
As a method for refining crystal grains of a hot-rolled steel material, for example, it is known to carry out a high rolling reduction, especially in the subsequent stage of hot finish rolling, refining austenite grains and causing accumulation of rolling strains within the grains; and thereby to refine the ferrite grains obtained after cooling (or after transformation). Further, in view of facilitating the ferrite transformation by inhibiting recrystallization and recovery of the austenite grains, it is effective to cool a steel sheet to below a predetermined temperature (e.g. 720° C. or below) within a short period of time after rolling. That is, in order to manufacture a hot-rolled steel sheet with fine crystal grains, it is effective, subsequent to hot finish rolling, to rapidly cool a steel sheet after rolling, by arranging a cooling apparatus capable of cooling more quickly after rolling than ever before.
Several techniques which enable manufacturing of a hot-rolled steel sheet having fine crystal grains, or several techniques which are applicable to manufacturing a hot-rolled steel sheet having fine crystal grains have been disclosed. For example, Patent Document 1 discloses a manufacturing method of a hot-rolled steel sheet having ultra fine crystal grains, wherein a hot-rolled steel sheet is manufactured by performing multi-pass hot rolling of a steel sheet or a slab consisting of a carbon steel or low-alloy steel containing 0.01% to 0.3% C by mass; a final rolling pass is completed at a temperature above Ar3 point; and then cooling is performed to 720° C. or below, within 0.4 second. Patent Document 2 discloses manufacturing equipment of a hot-rolled steel sheet comprising: a final stand of a row of hot finish rolling mills; a first cooling apparatus; a second cooling apparatus; and a coiling apparatus, which are disposed in the mentioned order in a transporting direction of a steel sheet, wherein a non-cooling region is provided between the first cooling apparatus and the second cooling apparatus; the first cooling apparatus comprises: a nozzle which forms an impact region of a belt-like or ellipse-shaped jet on a surface to be cooled of the steel sheet; and a damming roll, which dams up the cooling water sprayed from the nozzle; and the damming roll is arranged in a manner that a pool of cooling water is formed in a region between a roll in the final stand and the damming roll, and that the steel sheet being transported in the first cooling apparatus is immersed into the pool of cooling water. Further, Patent Document 3 discloses hot-rolling equipment of a steel sheet, wherein cooling equipment for supplying cooling water onto an upper surface of a steel sheet while passing the steel sheet, is disposed at a position close to an entry side and/or an exit side of a rolling mill which hot-rolls the steel sheet; the cooling equipment comprises a header which has a nozzle configured to spray rod-like water over the upper surface of the steel sheet at an angle of depression in the range of 30° to 60° toward the rolling mill; and the header is positioned such that the cooling water supplied to the steel sheet is retained by the work rolls in the rolling mill. Patent Document 3 also discloses that a distance between a tip of an upper nozzle and a pass line is preferably set in the range of 500 mm to 1800 mm in order to avoid a situation that the cooling water becomes diffused and loses its rod-like shape; and that the effects of retaining the cooling water is deteriorated.